The present invention relates to a pneumatically operated screw driver providing an axially driving force by a piston and rotational force by a pneumatic motor for screwing a threaded fastener into a woody member or the like.
U.S. Pat. No. 6,026,713 discloses a pneumatically operated screw driver including a driver bit engageable with a groove formed in a head of the fastener. The driver bit is connected to a piston which is driven in an axial direction of the driver bit upon application of a pneumatic pressure to one side of the piston. Further, a pneumatic motor is provided for rotating the piston about its axis. Thus, the driver bit is axially movable while being rotated about its axis for screwing the fastener into a target. Further, a bumper is provided so as to absorb kinetic energy of the piston moving to its bottom dead center. An operation valve associated with a trigger is provided for opening a main valve in order to apply pneumatic pressure onto the piston.
The disclosed screw driver also includes a return chamber to which a compressed air is accumulatable for applying compressed air to the piston in order to move the piston and the driver bit to their initial positions. More specifically, accumulation of the compressed air into the return chamber is started when the piston is about to reach its bottom dead center. When the screw fastening operation is terminated upon abutment of the piston onto the bumper, the compressed air accumulated in the return chamber will be applied to an opposite side of the piston so as to return the piston and the driver bit to their original positions. When the operation valve is returned due to release of the trigger, the main valve is closed, so that the compressed air applied to the upper surface of the piston is discharged out of the frame. Thus, the piston and the driver bit are moved to their initial top dead center positions because of the application of the compressed air supplied from the return chamber to the lower surface of the piston.
U.S. Pat. No. 6,073,521 discloses a pneumatically operated screw driver in which a throttle is provided at an air passage between the main valve controlling a supply of the compressed air and the operation valve controlling the main valve. Because of the throttle, a timing of restoring the main valve to its initial position in response to the closing operation of the operation valve can be retarded. The closing operation is done by releasing the trigger. By the retard, rotational movement and axial movement of the driver bit still continues for a predetermined period, ensuring screw fastening operation.